This invention relates to the composition of electrically semi-conductive devices having point-to-point electrical resistance that increases with increasing temperature as well as to a unique method for manufacturing such a semi-conductive composition as well as specific devices utilizing such a composition.
As pointed out in U.S. Pat. Nos. 3,435,401, 3,793,716, 3,823,217, 3,861,029, and 3,914,363, electrically conductive thermoplastic compositions have been prepared in the prior art by the addition of conductive carbon black to a polymeric base. The theory of operation of such compositions whereby such compositions provide a current limiting or positive temperature coefficient function has been thoroughly described. Moreover, the use of such self-regulating semi-conductive compositions and products using such compositions has been thoroughly described as having a large variety of uses ranging from electric heating to heat sensing and circuit breaker type applications. In each such use, however, it has been pointed out the disadvantage of the use of high carbon black loadings in connection with such products, such disadvantages including inferior elongation characteristics as well as inferior stress and crack resistance. While it is well known that semi-conductive thermoplastic compositions will show a resistivity rising with temperature, such compositions have also shown negative temperature co-efficients which accompany use of semi-conductive composition above that temperature at which the polymer will melt.
It is clear, however, that all of the prior art teachings known to applicant have dealt specifically with the utilization of what is referred to as low volume resistivity carbon blacks such as are described in the Cabot Corporation's Pigment Black Technical Report S-8 entitled "Carbon Blacks For Conductive Plastics". A typical conductive carbon black in extensive use is Cabot Vulcan XC72, an oil furnace black having a critical volume resistivity occurring at or about 15% by weight of the carbon black in the basic matrix. Moreover, the prior art assumes that electrically conductive thermoplastic compositions shall use such highly conductive carbon blacks and therefore much effort has been addressed to related issues of physical properties resulting from use of such carbon blacks in varying densities.